1. Field of the Invention
The present invention relates to pressure pulse generators in general. In particular, the invention to pressure pulse generators such as the "mud siren" or "fluid siren" type used in oil industry MWD (Measurements-While-Drilling) operations to transmit downhole measurement information to the well surface during drilling by way of a mud column located in a drill string as well as those used in flow measurement systems.
2. Description of the Related Art
Many systems exist for transmitting data representative of one or more measured downhole conditions to the surface during the drilling of a well borehole. One such system, described in Godbey U.S. Pat. No. 3,309,656, employs a downhole pressure pulse generator or modulator and is operated to transmit modulated signals carrying encoded data at acoustic frequencies to the surface by way of the mud column in the drill string. In such a system, it has been found useful to power the downhole electrical components by means of a self-contained mud-driven turbine generator unit (known as a "mud turbine") positioned downstream of the modulator.
Existing pressure pulse generators of the mud siren type usually take the form of "turbine-like" signal generating valves positioned in the drill string near the drill bit and exposed to the circulating mud path. A typical modulator includes a fixed stator and a motor-driven rotatable rotor, positioned coaxially of each other. The stator and rotor are each formed with a plurality of block-like radial extensions or lobes spaced circumferentially about a central hub so that the gaps between adjacent lobes present a plurality of openings or ports to the oncoming mud flow stream. When the respective ports of the stator and rotor are directly aligned, they provide the greatest passageway for flow of drilling mud through the siren. When the rotor rotates relative to the stator, alignment between the respective ports shifts, interrupting the flow of mud to generate pressure pulses in the nature of acoustic signals. Rotation of the rotor relative to the stator in the circulating mud flow produces a cyclic acoustic signal that travels up the mud column in the drill string and is detected at the drill site surface. By selectively varying the rotation of the rotor to produce changes in the signal, modulation in the form of an encoded pressure pulse is achieved which carries information from downhole instruments to the surface for analysis.
Recently, fluid sirens have been developed in which the rotor is driven by fluid flow rather than by a motor. These fluid sirens are useful for transmitting data relating to fluid flow rates and fluid densities. An example of such a siren is described in greater detail in commonly assigned U.S. patent application Ser. No. 08/404,232, which is co-pending.
The lobe configuration and the relative placement of the stator and rotor elements of fluid sirens of this nature subject the rotor to fluid dynamic forces from the fluid stream that cause the rotor to seek a "stable closed" position in which the lobes of the rotor block the ports of the stator. There is, thus, an undesirable tendency for the modulator to assume a position that blocks the free flow of fluid. This increases the likelihood that the siren will jam, as solids carried by the mud or other fluid stream are forced to pass through restricted siren passages. In commercial MWD operations, however, the spacing between the rotor and stator components of the siren must be narrow in order to produce satisfactory acoustic signals. This requirement makes the siren particularly susceptible to jamming or obstruction by solids present in the fluid stream.
The jamming problem often occurs when the rate of fluid flow is low. If the flow rate is low, the rotor may turn slowly, or not at all, raising the specter that particles will become lodged in the siren. Jamming also occurs when the fluid flow rate is very high and turbulent, causing the siren to lock up. Prolonged siren closure can obstruct mud flow to such an extent that lubrication of the drill bit and other vital functions of the mud become so adversely affected that the entire drilling operation is jeopardized.
A number of approaches have been proposed to solve the problem caused by the tendency of sirens to assume the closed position described above. One such approach, described in Patton, et al., U.S. Pat. No. 3,792,429, is to use magnetic force to bias the siren toward an open position and hold it there in the event the rotor becomes inoperative. Magnetic attraction between a magnet attached to the siren housing and a cooperating magnetic element positioned on the rotor shaft develops sufficient torque to overcome the fluid dynamic torque caused by the drilling mud stream. This approach has the disadvantage that introduction of an extraneous magnetic field downhole can interfere with measurements of the earth's magnetic field (used to derive tool orientation). It also requires more power from the drive motor to overcome the effects of magnetic forces tending to resist rotation.
Unfortunately, none of the methods or devices developed to date has been entirely successful in eliminating the problem of siren rotor lock-up or stalling, particularly when the flow rate through the siren is very high or very low. The maximum or minimum flow rate at which the rotor will either lock-up or stall is a function of the specific siren design, and is related to the pipe diameter, fluid viscosity, efficiency of the driving turbine and the inherent friction of the siren unit.
A related problem exists with fluid powered sirens used as flowmeters when subjected to excessively high flow rates. With existing siren designs, a high flow rate causes the rotor to spin faster than desired. High frequency signals have lower amplitudes. Also, signals with very high frequencies tend to attenuate rapidly over distances. Therefore, signals with high frequencies often are undetectable by surface detection equipment. It has been shown that the amplitude of a pulsed pressure signal decreases as the pulse frequency increases. Generally, a pressure pulse produced in a long pipe will lose up to 50 percent of its amplitude when the frequency is increased from 1 Hz to 10 Hz and will lose up to 75 percent at 30 Hz. Also, there are other factors that affect pulse amplitude, such as the pipe diameter, pipe length and kinematic viscosity of the fluid.